Add `ItemCollection.to_collection`

[gadomski]

This would be a nice convenience method that would generate the extents, etc from the items in the ItemCollection.

[jsignell]

I think of ItemCollections as more analogous to Catalogs than to Collections. I guess I think that because you can represent items from more than one Collection in an ItemCollection. But I guess from pystac's perspective Collections just inherit from Catalogs so maybe it doesn't matter?

[gadomski]

Yeah, I agree with the general point that not all ItemCollections map to a Collection. However, often they do (e.g. a search result), and creating a "sub-collection" (here's a collection that describes my search results) could be of use to some folks?